1. Field of the Invention
The present invention relates to switching regulator circuits and in particular to switching regulator circuits which use slope compensation techniques to stabilise the regulator.
2. Description of the Related Art
Switching regulators are very commonly used in DC-DC conversion as they offer higher efficiency than linear regulators. They typically consist, in their most basic form, of an inductor, a capacitor, a diode and a switch which switches the inductor alternately between charging and discharging states. These basic elements can be arranged to form a step-down (buck), step-up (boost) or inverting buck-boost regulator.
Control of the switch has been achieved previously by techniques such as “voltage mode control” and “current-mode control”. In the case of a basic constant frequency current-mode control buck converter the switch is connected to an input voltage and is closed at the beginning of a clock cycle. Closing the switch causes the current in an inductor connected between the switch and the output of the converter to rise. This current is monitored and compared against the output of an error amplifier. When the output voltage of the inductor current monitor exceeds the output voltage of the error amplifier the switch is turned off, and not turned on again until the beginning of the next clock cycle. In this way the output voltage is controlled to the required value.
It is a well known phenomenon in current-mode control regulators that when the duty cycle (switch ON time/clock period) of the converter exceeds 50%, subharmonic oscillation can occur. Subharmonic oscillation is an undesirable repeating pattern in inductor current that occurs every two or more periods. [See R. W. Erickson, D. Maksimović, Fundamentals of Power Electronics, 2nd Edition, Kluwer Academic Publishers, 2001, pp. 439-449.]
It is also well known that the problem of subharmonic oscillation can be addressed by using the technique known as “slope (or ramp) compensation”. This is typically done by adding a predetermined duty-cycle-independent synchronous sawtooth signal (variously termed an artificial ramp, a compensatory ramp or loosely but simply “additional slope”) to the measured inductor current up slope, or alternatively by subtracting a similar signal from the error amplifier output. To ensure stability for all duty cycles up to 100%, the slope of this ramp should be equivalent to at least half of the anticipated maximum inductor current down slope.
DC-DC converters usually have some means to limit the current in the inductor. There are many different ways to implement the current limit, but it is often convenient to use the signal of “inductor current plus additional slope” to give a current limit. Examples of this are common and one way of doing this is to simply limit (“clamp”) the voltage output of the error amplifier. In that case the error amplifier signal is not only used for control of the converter but also for current limiting. One drawback of this is that due to the “additional slope” the current limit has different values for different duty cycles when using a fixed voltage limit on the output of the error amplifier. This means that the true maximum current in the inductor falls proportionally with the duty cycle.
U.S. Pat. No. 6,611,131 discloses a technique to remove the effect of the “additional slope” for the current limit by increasing the current limit by the same amount as the slope on a cycle-by-cycle basis by means of an adjustable voltage clamp circuit controlled by the slope compensation circuit. This, however, has the same problem as having no slope compensation at all for the current limit comparison, in that subharmonic oscillation will occur in current limit.